LONDON — If Will and Kate's first child is a girl, it's now clear that she'll probably become queen one day — and not even getting a little brother can mess that up.

The Commonwealth countries agreed Friday to change centuries-old rules of succession that put sons on the throne ahead of any older sisters. So that hypothetical daughter of Prince William and Kate Middleton — now known as Catherine, Duchess of Cambridge — would have a prime place in history: the first princess to beat out any younger brothers and accede to the throne.

Had these rules been in place in the 1500s, Henry VIII would have just been a rather large historical footnote.

The move is a baby step: Before taking effect, the changes still must be approved by the legislatures of the 16 nations where Queen Elizabeth II is head of state. Still, the agreement, which was reached at a meeting of Commonwealth nations in Perth, Australia, represents a triumph over practices now considered outdated and sexist in much of the world.

Nations including Sweden, Belgium, the Netherlands and Norway have already taken similar steps.

Will and Kate's lavish April wedding renewed a decades-long debate over succession.

Middleton told a well-wisher in Canada this summer that she hopes to start a family. William has said the same.

Once their honeymoon was over, baby talk started, adding urgency to the dialogue, although officials insist that talk of a pregnancy is premature.

Historians think it's about time.

"You shouldn't muck around too much with the constitution, but it's a good idea to change this at this time," said royal expert Hugo Vickers. "It's much better to have it sorted out before any babies come along."

The new rules would only apply to future heirs and would have no impact on the current line of succession.

William is second in line to the throne after his father, Prince Charles, who is the queen's firstborn child. Charles' sister, Anne, is lower in the line of succession than her younger brothers Andrew and Edward by virtue of their male gender.

Charles had only sons, William and Prince Harry, so the issue of gender was never raised.

In 2009, the government of then-Prime Minister Gordon Brown considered a bill that would end the custom of putting males ahead of females in the succession line. It also would lift a ban on British monarchs marrying Roman Catholics. The government did not have time to pursue it before Brown left office.

The rule has kept women from succeeding to the throne in the past. Queen Victoria's first child was a daughter — also called Victoria — but it was her younger brother who became King Edward VII.

If Queen Victoria had been able to pass her crown to her firstborn, Britain's Princess Victoria would have had a brief reign before her death in 1901.

That would have made her son — Wilhelm II, who at that time was the German Kaiser — king. With Wilhelm II ruling both Germany and Britain, there may not have been two world wars.

As king, Henry VIII set in motion the creation of the Church of England. His six marriages left an insecure succession — one sickly son and two princesses, according to the monarchy's official website. Charles I's reign in the 17th century led to a bloody civil war.

Prince William and his wife have been credited with freshening up a staid monarchy, and new succession rules seem to fit right in.

"In this day and age, why should a royal son be more important than a royal daughter?" said Joe Little, managing editor of Majesty magazine.

The same goes for the decision Friday in Perth to lift a ban on monarchs marrying Roman Catholics. Critics had called the rule blatantly discriminatory since royals are free to wed Jews, Muslims, Hindus or members of any other religion."Britain is no longer the religious country that it once was," Little said. "While not denigrating the importance of religion, it plays much less of a role now then it did 60 years ago."

"The pope is responsible for some horrors," said Anna Marsh, 73, who was cycling in London.

Her biking buddy Jill Gregory, 71, was fine with the idea — and also fully in favor of giving firstborn girls an equal right to the throne.

"In terms of ability, I don't think women are any different than men," Gregory said, pointing to the queen and her late mother.

Elizabeth II succeeded her father, King George VI, because he had no sons. If she had had a younger brother, he would have jumped above her in the line of succession.

Prime Minister David Cameron had pushed for the changes, calling it a matter of equality.

New Zealand will now chair a working group of Commonwealth countries to discuss how to accomplish the reforms. It's not a simple process. Getting all 16 countries to begin the legislative changes is what has held them up for decades.

However long it takes, Patricia Wager of London said it would clear up something that should not be an issue in the modern world.

King Jigme of Bhutan and Jetsun Pema

When people mention a commoner marrying into royalty, the most recent couple to come to mind may be Prince William and Kate Middleton but they're not the only royal/commoner pair to cause a stir. Here are a few other couples that found love outside their social class.
King Jigme Khesar Namgyel Wangchuck of Bhutan holds the hand of Queen Jetsun Pema, a 21 year old student at London's Regent College, after their wedding on October 13, 2011.
(Paula Bronstein / Getty Images)
ShareBack to slideshow navigation

Prince William and Kate Middleton

Britain's Prince William married Kate Middleton, a commoner with coal-mining roots, on Friday, April, 29, 2011 at Westminster Abbey in London. Upon their nuptials, the couple were titled the Duke and Duchess of Cambridge.
(Alastair Grant / AP)
ShareBack to slideshow navigation

Duchess of Alba and Alfonso Diez Carabantes

Maria del Rosario Cayetana Fitz-James-Stuart, the Duchess of Alba, married for the third time at age 85 on October 5, 2011 in Seville, Spain. Her new husband is Alfonso Diez Carabantes, a 61-year-old civil servant.
(José Manuel Vidal / Pool via Getty Images)
ShareBack to slideshow navigation

Prince Albert II and Charlene Wittstock

Prince Albert II of Monaco and one-time Olympic swimmer Charlene Wittstock were married on July 1, 2011. The couple met when Charlene traveled to Monaco for a swimming competition in 2000.
(Pascal Le Segretain / Getty Images)
ShareBack to slideshow navigation

Princess Victoria and Daniel Westling

Crown Princess Victoria of Sweden and her husband Prince Daniel meet the general public as they appear on the Lejonbacken Terrace after their wedding ceremony on June 19, 2010 in Stockholm, Sweden. The couple fell in love while Daniel served as Victoria's personal trainer.
(Pascal Le Segretain / Getty Images)
ShareBack to slideshow navigation

Princess Sayako and Yoshiki Kuroda

Princess Sayako Kuroda of Japan married Yoshiki Kuroda, a commoner, on November 15, 2005 and lost her royal title as a result.
(Issei Kato / Pool via AFP - Getty Images)
ShareBack to slideshow navigation

Crown Prince Felipe and Letizia Ortiz Rocasolano

Spain's Crown Prince Felipe kept his relationship with journalist Letizia Ortiz Rocasolano under wraps until they announced their engagement in 2003. They were married on May 22, 2004.
(Heribert Proepper / AP)
ShareBack to slideshow navigation

Crown Prince Haakon and Mette-Marit Tjessem Høiby

Crown Prince Haakon married single mother Mette-Marit Tjessem Høiby on August 25, 2001. Mette-Marit's wild past in Oslo caused some concern among the royal family and general public but their opinion of her rose after she admitted to her past wrongs a week before the wedding.
(Slaven Vlasic / Getty Images)
ShareBack to slideshow navigation

King Abdullah II and Rania al Yassin

Jordan's King Abdullah II and Rania al Yassin met at a dinner party in January 1993 and were engaged two months later. The two married on June 10, 1993.
(Nasser Ayoub / Royal Palace via AP)
ShareBack to slideshow navigation

Crown Prince Willem Alexander and Máxima Zorreguieta

Crown Prince Willem Alexander of the Netherlands and Máxima Zorreguieta, an investment banker, married on March 30, 2001.
(Carl De Souza / AFP - Getty Images)
ShareBack to slideshow navigation

Crown Prince Frederik and Mary Donaldson

Crown Prince Frederik and Crown Princess Mary of Denmark wave to well-wishers from the balcony at Amalienborg castle in Copenhagen after their wedding ceremony at Copenhagen's cathedral on May 14, 2004. Mary, an Austrailian marketing consultant, met Frederik at the 2000 Summer Olympics in Sydney.
(Odd Andersen / AFP - Getty Images)
ShareBack to slideshow navigation

King Hussein and Elizabeth Najeeb Halaby

Born to an Arab-American family, Elizabeth Najeeb Halaby met King Hussein while working in Jordan on urban planning and design projects. She converted to Islam upon marrying Hussein on June 15, 1978 and changed her name to Noor al-Hussein. The two had a happy marriage until the King died of cancer in 1999.
(Rabih Moghrabi / AFP - Getty Images)
ShareBack to slideshow navigation

Prince Andrew and Sarah Ferguson

Prince Andrew, the Duke of York, married Sarah Ferguson on July 23, 1986. The marriage didn't last and after four years of separation the couple filed for divorce in 1996.
(Tim Graham / Getty Images)
ShareBack to slideshow navigation

Prince Rainier III and Grace Kelly

Prince Rainier III of Monaco met American screen legend Grace Kelly during a photo shoot in 1955. The two married a year later on April 19, 1956 in what the press called "The Wedding of the Century." They remained happily married for almost 30 years until Grace had a stroke while driving, resulting in a deadly car accident.
(Getty Images)
ShareBack to slideshow navigation